Nucleoside reverse transcriptase inhibitors (NRTIs) are among the most potent antiretrovirals used clinically, and are often used in first-line therapy for HIV infection. However, resistance is increasingly common in HIV drug experienced patients, and there is an urgent need to identify and develop new antiretrovirals active against these resistant HIV strains. All approved NRTIs act as chain terminators because they lack a 3'OH, and it has been a long standing paradigm that the absence of the 3'OH is essential for antiviral activity. However, this feature can also impart detrimental properties to the inhibitor, such as reduced affinity for RT compared to dNTP substrates, as well as reduced intracellular conversion to the active nucleoside triphosphate. We and our collaborators have obtained data with the novel nucleoside 4'-ethynyl, 2-fluoro deoxyadenosine (4'E-2FdA) that challenge this existing paradigm. 4'E-2FdA is the most potent NRTI described to date and acts as a chain terminator despite retaining an accessible 3'OH. Our preliminary data suggest that this apparent chain termination arises from difficulty of the primer 3'-terminus to translocate following incorporation of the compound. We therefore propose that 4'E-2FdA is a Translocation-Deficient Reverse Transcriptase Inhibitor (TDRTI). We hypothesize that the presence of the 3'OH, 4'E and 2F groups contribute to the high potency and result in the novel mechanism of inhibition. We propose to conduct detailed biochemical studies to better understand how these novel NRTIs work and to determine the specific characteristics of these compounds that contribute to their pronounced antiviral potency and excellent resistance profiles. To this end we will pursue the following Specific Aims: 1. Determine the biochemical mechanism of RT inhibition by TDRTIs. 2. Determine the biochemical mechanism of TDRTI excision by RT. 3. Determine inhibition of clinically relevant NRTI-resistant RTs by TDRTIs;interactions of clinically relevant RT inhibitors with TDRTIs and toxicity of combinations. 4. Determine the mechanism of HIV resistance to TDRTIs. Addressing these aims should significantly advance scientific knowledge and be invaluable in the design of new generations of highly active innovative NRTIs. PUBLIC HEALTH RELEVANCE: This project will characterize the biochemical and molecular basis for the unprecedented efficiency of a novel class of compounds that suppress HIV viruses extremely efficiently, and by doing so, it will help develop anti- HIV therapeutics that are both less susceptible to current clinically significant resistance mutations as well as more refractory to the development of viral drug resistance.